Crunch build failed due to "ClassNotFound" in doxia.
This is caused by maven-project-info-reports-plugin updated to 3.0.0, depends on
doxia-site-renderer 1.8 (which has org.apache.maven.doxia.siterenderer.DocumentContent
this class), while maven-site-plugin:3.3 depends on doxia-site-renderer:1.4 (which
doesn't have org.apache.maven.doxia.siterenderer.DocumentContent)
Specify maven-site-plugin to 3.7 can resolve this.

This fixes the existing call for numReducers as it was not working as
intended for non-memory PCollections due to using an invalid amount
of numReducers. To increase flexibility when using the API,
another call was added that allow to directly pass the GroupingOptions.

Remove lambda support from crunch-core, and instead implement a new module called crunch-lambda.
This will allow full use of Java 8 features in implementing support for lambda expressions and
method references, without requiring a dependency on Java 8 for crunch-core. Pthings are wrapped
into analagous Lthings which can be operated on with an API inspired both by the existing Crunch
API and the Java 8 streams API.